If you can get ANY reply from a website, then the firewall is not the issue --
it seldomly is but everyone likes to finger point the firewall --
imo, that's a lack of understanding. If ALL access to ANY website fails, then investigate
the firewall allowing port 80 outbound.

Most frequently, a browser will have two kinds of problems

site can not be found

pages don't load properly or completely

On (1), we have many cases here on Techspot of browsers being hijacked,
ie: altered by some virus such that URLs from the user get redirected elsewhere or
fail to produce a display.

On (2), the users browser cache is contaminated (needs to be flushed), OR
even the remote website can have issues and not completely transmit the entire content requested.

Many users have far too many ActiveX components loaded and some have too many LSP hooks -- both of which can impact browser functionality.